1. Field of the Invention
The present invention relates, in general, to a scanning apparatus for a laser printer and, more particularly, to a scanning apparatus, which is formed to allow a light modulator to be non-perpendicular to the shaft of a photosensitive drum, which is a scanning object, thus performing scanning at a higher resolution and speed within the allowable limits of other devices.
2. Description of the Related Art
A light beam scanning apparatus recently used in a laser printer is a device for scanning light beams, forming spots on a photosensitive medium with respect to the light beams and forming an image, in an image formation device, for example, a laser printer, a display device, a Light Emitting Diode (LED) printer, an electronic picture copier and a word processor.
With the development of image formation devices towards miniaturization, high speed and high resolution, the light beam scanning apparatus has been steadily researched and developed to be miniaturized and have high speed and high resolution characteristics to cope with the development of the image formation device.
Among recently popularized printers, a laser printer has attracted attention due to printing speed and quality higher than that of an ink-jet printer in the monochrome printer field.
FIG. 1 is a view showing a conventional scanning apparatus using a light modulator for a laser printer.
Referring to FIG. 1, if a laser diode 11 generates a laser beam, a collimator lens 12 converts the laser beam into collimated light, and converges the collimated light onto a multi-beam control light modulator 13.
The multi-beam control light modulator 13 diffracts and modulates the laser beam, converted into the collimated light, and outputs a plurality of (N) beams. A projection lens 15 converges the plurality of diffracted beams in the direction of the shaft of a rotating mirror 14.
In this case, if a slit 18 is disposed between the multi-beam control light modulator 13 and the rotating mirror 14, the slit 18 selectively passes therethrough beams having desired properties among the beams diffracted by the multi-beam control light modulator 13, thus transmitting the selected beams to the rotating mirror 14.
The diffracted beams, converged in this way, scan a drum 17 or a scanning object using a polygon mirror that moves at a constant linear velocity, or using a Galvano mirror that moves at a non-constant linear velocity.
At this time, the rotational speed of the rotating mirror 14 can be decreased in proportion to the number of beams output from the light modulator 13.
Accordingly, if the rotating mirror 14 is implemented as a polygon mirror, an F-θ lens 16 deflects the diffracted beams, reflected from the polygon mirror and moved at a constant angular velocity, in a main scanning direction, corrects aberration of the diffracted beams, and focuses and irradiates the aberration-corrected beams onto the surface of the photosensitive drum 17 or the scanning object.
If the rotating mirror 14 is implemented as a Galvano mirror, the F-θ lens 16 deflects the diffracted beams, reflected from the Galvano mirror and moved at a constant angular velocity, in a main scanning direction, corrects aberration of the diffracted beams, and focuses and irradiates the aberration-corrected beams onto the surface of the photosensitive drum 17 or the scanning object.
FIG. 2A illustrates a procedure of printing on paper performed by the scanning apparatus of FIG. 1.
The light modulator 13 has a structure in which 1080 mirror cells constituting a mirror cell array are typically arranged in a line. The scanning apparatus of FIG. 1 is constructed to allow the light modulator 13 to be perpendicular to the shaft of the photosensitive drum 17, so that the light modulator 13 scans 1080 vertically arranged pixels at one time on the photosensitive drum 17 while moving in a horizontal direction, as shown in FIG. 2A. If the horizontal length of a sheet of paper is 10,000 pixels, 1080×10,000 pixels are printed only when the light modulator 13 operates 10,000 times while crossing the paper along the horizontal length.
However, in this scheme, the light modulator 13 must operate a number of times corresponding to the number of pixels corresponding to the horizontal length of the paper in order to print a single line. However, the operational speed of the light modulator 13 is currently limited to a maximum of several hundred kHz. Therefore, the increase in scanning speed using this scheme is restricted by the limit to the operational speed of the light modulator 13.
Technology modifying the scanning apparatus of FIG. 1 and allowing the mirror cell array of the light modulator 13 to be parallel to the shaft of the photosensitive drum 17 is also well known. In this case, 1080 pixels are printed at one time in a horizontal direction. FIG. 2B illustrates a scanning process according to this scanning method.
However, in this scheme, 1080 pixels must be printed at one time in a horizontal direction, so that the rotational speed of a polygon mirror for reflecting the pixels must be very high. However, since the rotational speed of the polygon mirror is currently limited to about 20000 rpm, the increase in scanning speed using this scheme is restricted by the limit to the rotational speed of the polygon mirror 14.
As described above, the conventional vertical scanning scheme is fundamentally restricted by the limit to the operational speed of the light modulator, and the conventional horizontal scanning scheme is fundamentally restricted by the limit to the rotational speed of the polygon mirror. Therefore, a method of further increasing scanning speed while maintaining resolution within the allowable limits of the current rotational speed of the polygon mirror and the current operational speed of the light modulator is required.
In relation to this scheme, U.S. Pat. No. 6.025,859 discloses a laser printer using two light modulators to increase printing speed. However, the laser printer cannot fundamentally solve the above-described restrictions on printing speed.